Sounds like you have a 3500 4x4. There should be a grease fitting on the u-joint cross. It's not a zirc but one of those shallow, 1/8 inch diameter fittings that is concave with a ball detent. It accepts a special thin shaft grease extension with a tiny tapered end to press the ball and deliver the grease in there. You attach it to the end of your standard size grease gun hose (the attachment has a built in zirc fitting). Lubrimatic makes it.

That being said, while rotating the joint to find it, you'll need a flashlight or shop light aimed in there to see it. It's such a confined area in there that even with the special fitting, it's hit or miss getting it to line up, press hard enough to push the ball and keep it in there to deliver a few shots of grease. It will test your patience but it can be done. When it comes time to replace those u-joints, go with high quality lube type Spicer joints with normal zircs on the end caps.

At least that joint can more or less be lubed. As much as I love my Rams over the years, Chrysler continues to stupidly use non-greasable driveshaft and front axle shaft u-joints in addition to upper and lower ball joints on the 2500 and 3500 4x4's. They can dry up in as few as 20k miles but most cash out around 40-60k miles. Plan on replacing them - use greasable Moogs for the ball joints and greasable Spicer u-joints were possible. It's no picnic when a crispy factory ball joint decides to cash out when your flying down the freeway at 70 mph! Same goes for the u-joints. On factory axle shaft u-joints, I've seen the cross end cap needle bearings virtually disintegrate into tiny particles.